WASHINGTON — The United States would "take out" any North Korean missile seen to be heading for American soil, Defense Secretary Jim Mattis said Monday.

He declared that any such North Korean attack could lead to war.

Responding to reporters' questions about North Korea's assertion that it might soon fire four ballistic missiles into the waters off Guam, a U.S. territory in the Pacific, Mattis made clear he did not want to be seen as escalating the tensions.

"If they fire at the United States, it could escalate into war very quickly," he said. "Yes, that's called war, if they shoot at us."

Later he said, "If they shoot at the United States, I'm assuming they hit the United States. If they do that, it's game on."

Mattis said U.S. missile detection and tracking systems can determine swiftly whether a missile launched from North Korea is headed for U.S. soil. North Korea said last week it is considering launching four missiles to land just short of Guam.

If a missile is judged to be headed for the island, Mattis said: "We'll take it out."

If the U.S. determines the missile would fall into the sea short of Guam, he said "it becomes an issue we take up however the president chooses."

The U.S. has missile defenses on Guam, at sea and in the continental U.S. that are designed to shoot down ballistic missiles.

Mattis was asked whether decisions had already been made about how to respond in the event a North Korean missile lands in the waters off Guam.

"You can't make all those kinds of decision in advance," he said. "There's a host of things going on. There's allies that we consult with."

Mattis was reluctant to speak in detail about how the Trump administration will handle future North Korean missile launches.

"I need a certain amount of ambiguity on this" to prevent North Korea from knowing too much about U.S. planning, he said.